The description of this trophy is misleading, as you start the chapter with a shotgun and a SMG, but rest assured, the Skorpion classifies as a handgun. Be careful with your shots as you progress through the level, get as many headshots as possible, and sprint through enemies to advance if you need to conserve ammo. A_Tennis_Giraffe gives us a good reminder as well, melee and grenades will void the trophy, so reload when you can! I played through on Normal, and completed the Operation with 22 rounds left, so there is room for some error.